Alloclasite, or (Co,Fe)AsS, is a sulfosalt mineral (IMA symbol: Acl).
Alloclasite crystallizes in the monoclinic system and typically forms as columnar to radiating acicular prismatic clusters.
It is an opaque steel-gray to silver-white, with a metallic luster and a black streak.
It is brittle with perfect cleavage, a Mohs hardness of 5 and a specific gravity of 5.91–5.95.
[3] Its name is derived from Greek for "other" and "to break," in reference to its distinct cleavage which distinguished it from the similar appearing mineral marcasite.